Rotating machines such as gas turbines have portions commonly referred to as rotors that rotate within stationary casing components, such as a shroud. Clearance dimensions must be maintained between the rotor and the shroud to prevent impacts between the components. This is a particular concern in gas turbines.
A gas turbine uses hot gases emitted from a combustion chamber to rotate a rotor, which typically includes a plurality of rotor blades circumferentially spaced around a shaft. The rotor shaft is coupled to a compressor for supplying compressed air to the combustion chamber and, in some embodiments, to an electric generator for converting the mechanical energy of the rotor to electrical energy. The rotor blades (sometimes referred to as “buckets”) are usually provided in stages along the shaft and rotate within a casing configuration, which may include an outer casing and an inner casing or shroud ring for each respective stage. As the hot gases impinge on the blades, the shaft is turned.
The distance between the tips of the blades and the shroud ring is referred to as “clearance.” As the clearance increases, efficiency of the turbine decreases as hot gases escape through the clearance. Therefore, clearance between the blade tips and the shroud should be minimized in order to maximize efficiency of the turbine. On the other hand, if the amount of clearance is too small, then thermal expansion and contraction of the blades, the shroud, and other components may cause the blades to rub the shroud, which can result in damage to the blades, the shroud ring, and the turbine in general. It is important, therefore, to maintain a minimal clearance during a variety of operational conditions.
Methods and systems are known that attempt to maintain an accurate clearance by directing bypass air from the compressor around the casing to reduce thermal expansion of the casing during operation of the turbine. For example, U.S. Pat. No. 6,126,390 describes a passive heating-cooling system wherein the airflow to the turbine casing from the compressor or combustion chamber is metered depending on the temperature of the incoming air so as to control the rate of cooling of the turbine casing, or even to heat the casing.
The conventional passive air-cooling systems, however, assume a uniform circumferential expansion of the rotor and/or shroud and cannot account for eccentricities that either develop or are inherent between the rotor and shroud. Eccentricities can develop as a result of manufacturing or assembly tolerances, or during operation of the turbine as a result of bearing oil lift, thermal growth of the bearing structures, vibrations, uneven thermal expansion of the turbine components, casing slippage, gravity sag, and so forth. Anticipated eccentricities must be accounted for in design and, thus, these eccentricities limit the amount of minimum designed clearance that can be achieved without rubbing between the blades and shrouds. The conventional approach to this problem has been to make static adjustments in relative position of the components during cold assembly to compensate for hot running eccentricity conditions. This method, however, cannot accurately account for the variations in eccentricities that develop during the operational life of the turbine.
Thus, an active alignment control system and method are needed to accurately detect and account for eccentricities that develop between turbine components over a wide range of operating conditions.